[Ron61]

Check the wiring, especially for grounds. If that is OK go to NAPA or some parts store and buy a gauge that you can use in your hand and hook it onto your sensor and see what it does. If it reads OK then you have a gauge problem or a wiring problem at the back of the gauge. If not your sensor could be going bad. By it acting the way it does when you first turn the key on I would suspect a wiring issue.

Ron